In the art of microcircuit designs and other electronic applications it was recognized some years ago that ferroelectric materials could be used much like capacitors in a dynamic random access memory (RAM). The term "ferroelectric material" is somewhat of a misnomer because many of these materials do not contain iron. However, the name implies that they possess ferroelectric properties analogous to their ferromagnetic counterparts. It was hypothesized early in the electronic memory industry that a nonvolatile random access memory with high performance and good economics could possibly be fabricated if a suitable ferroelectric material were found, such as discussed in "Ferroelectric Arrays: Competition for Core and Semiconductor Memories," pages 30-32, Digital Design. June 1973.
Thus, the art has searched for a ferroelectric material that was easily deposited, sputtered, or otherwise controllably applied to suitable substrates and would have properties compatible with the subsequent MOS processing or other electrical applications. It was recognized early that the thin film needed to be defect free, retain its polarity, and have good fatigue resistance. A significant potential advantage over current nonvolatile NMOS technology is that ferroelectric memory is known to have an endurance cycle of better than 10.sup.10 read and write cycles compared to 10.sup.6 -10.sup.7 cycles of the floating gate MOS technology.
Different ferroelectric materials have been proposed, such as potassium nitrate (KNO3), as discussed in the above-referenced Digital Design article and also in the March 1983 issue of Computer Design. Other materials proposed were PZT and PLZT. PZT is an acronym for a lead zirconate titanate ceramic. PLZT is an acronym for a ceramic of lead, lanthanum, zirconium and titanium Pb.sub.1 .sub.3x/2 La.sub.x (Zr.sub.y Ti.sub.1-y)O.sub.3. Another proposed material was lead titanate (Pb TiO.sub.3), as discussed in "Crystallization an Transformation of Distorted Cubic PbTiO.sub.3," page C-256, 7 J. Am. Ceram. Soc. 69, October, 1986.
Sputtering techniques have been investigated for applying these films. This technique suffers from the disadvantages that it requires expensive equipment, the composition deposited does not always correlate to the mixture utilized for the sputtering, which gives rise to quality control problems, and importantly, films deposited on certain substrates were not smooth and contained cracks and other deficiencies. Attempts to sputter PZT films onto silicon substrates, the most common substrate in the semiconductor industry, are characterized by microcracks probably resulting from different thermal coefficients between the silicon substrate and the sputtered PZT film, which, in order to achieve a useful thickness, required multiple sputtering cycles. Furthermore, the very low deposition rates in sputtering are not practical for commercial-scale production. Also, the high temperature necessary for sputtering is not desirable because it disturbs dopants in the substrate. Some investigation of sol-gel processing of PZT and PLZT to provide thin films is recorded in "Sol-Gel Processing of PbTiO.sub.3, PbZro.sub.3, PZT, and PLZT Thin Films", Brit. Cer. Proc. Vol., Vol. 36, 1985, pages 107-121. See also "Preparation of Ferroelectric PZT Films by Thermal Decomposition of Organometallic Compounds," pages 595-598, Journal of Materials Science, Vol. 19 (1984). This article reports some successes but the techniques used were deficient in several regards, such as the reported crystallization temperature was too high, resulting in the grain structure produced being too large for capacitor areas required in microelectronic memories, and some of the films had problems with cracking. A method which uses a lower temperature is desirable because diffusion of dopants in the substrate becomes significant if temperatures exceed 900.degree. C., and adhesion problems related to metal pads and interface occur above about 750.degree. C. Furthermore, the time required to produce a usable thickness was too long. Thus, for a long time there has been a need in the industry to find a method and composition to produce a ferroelectric material which in practice could be produced economically, applied to various substrates in the required purity and have physical characteristics necessary for use in thin films suitable in the manufacture of integrated circuits. Furthermore, not only was the development of suitable materials required, but also that a process be developed by which suitable materials could be reproducible and confidently applied to substrates having the desired composition as well as the desired adhesion, integrity, thickness, and other physical characteristics necessary for dependable performance for supporting commercial utilization.
A sol-gel refers to a composition which is made as a solution and then formed into a gel which forms an open lattice structure when it is dried. The perovskite crystalline class of ferroelectric ceramics, and more specifically lanthanum modified lead zirconate titanates, or PLZTs, have been known and used commercially in bulk form. Perovskites have a body centered cubic or pseudocubic crystal lattice structure in a general chemical formula (crystalline unit cell composition) of ABO.sub.3 where A is a metal cation a+2 (or +1 or +3) oxidation state, B is a metal cation in the +4 (or +3 or +5) oxidation state, and O is oxygen in a -2 state. Thus for PLZT, Pb or La occupy the "A" sites or the cubic (or pseudocubic) cell corners, the Zr or Ti occupies the "B" site at or near the cell center and oxygen is located in the cell faces. The present invention utilizes sol-gels to produce thin films in the PLZT family of films such as PLZT, PZT, and PLT compositions which can be utilized in fabricating ferroelectric thin films reproducibly and having required physical and chemical characteristics for reliable performance in electrical, as well as optical, applications. The present invention is advantageous in that PLZT, PLT and PZT films of varying compositions can be easily applied as thin films to suitable substrates by spin coating methods with equipment common in the semiconductor industry. These films can be applied with a high degree of quality control, and the composition of the applied film can be easily and reliably selected as desired using the method of the present invention.